Recently, the integration density of LSIs (Large Scale Integrated Circuits) has been increased, and caused the serious problem that the yield of satisfactory MOS (Metal Oxide Semiconductor) transistors which form devices, and the reliability are decreased due to the defective transistors. The defectiveness of MOS transistors will be caused due to, typically, breakdown of insulating gate oxide films and excessive leak current in junctions. The latter gives rise to the loss of information called poor refreshment in DRAM (Dynamic Random Access Memory). These causes for unsatisfactory MOS transistors are ascribed to not only the foreign particles on the surface of the silicon substrate in which devices are built up, but also crystal defects present in the region near the surface. The region near the surface of the silicon substrate is specifically in the range from the substrate surface to about 0.5 .mu.m deep, and this region is a portion of the substrate that is converted into an oxide insulated layer (silicon oxide film) in the production process for devices such as LSIs or into a depletion layer when a metal thin film is deposited on the substrate surface. The crystal defects located within this portion form structural defects in the silicon oxide film, and causes insulation breakdown in the LSI operation. The defects in the depletion layer causes a large amount of leak current to occur. On the contrary, crystal defects located in the region deeper than the near-surface region, particularly, oxygen precipitates, have the effect (gettering effect) to catch metal ions mixed into the silicon substrate in various production processes, preventing the near-surface region of the substrate from being contaminated with metal.
Therefore, for the quality control of silicon substrates in which LSIs are formed, it is necessary to provide an inspection apparatus capable of not only detecting foreign particles on the substrate surface, but also selectively detecting crystal defects located in the near-surface region.
Moreover, when liquid crystal displays which are formed of thin film transistors (for example, amorphous silicon thin films) are produced under quality control, it is essential to detect the foreign particles on the surface of a transparent substrate on which thin films are formed, the foreign particles or defects present in the near-surface region, and the foreign particles which are mixed into the interior of the thin films in the thin film depositing process or which adhere to the substrate surface.
There is proposed an apparatus for detecting foreign particles or defects on or below the surface of a sample by irradiating S-polarized light and P-polarized light on the sample surface, as is disclosed in Japanese Patent Laid-open JP-A-63-12943 (or U.S. Pat. No. 4,893,932 associated therewith). This apparatus detects the foreign particles or defects located on the sample surface by utilizing the fact that the intensity distributions of the irradiation light scattered by the foreign particles and defects on the sample surface are different depending on the polarization direction. However, the specification disclosing the apparatus of the invention does not specifically describe any means for detecting the foreign particles or defects present on the lower side (namely, in the medium of which the refractive index is larger than that of air) of the sample surface.
As disclosed in Japanese Patent Laid-open JP-A-5-264468, another apparatus is proposed for irradiating on a tested object first light of a wavelength for extracting relatively more optical information of the object surface and second light of a wavelength for extracting relatively more optical information of the interior of the object, and observing the optical images scattered (scattered images) by the defects on the surface or within the interior of the object in association with the first wavelength and second wavelength. This apparatus determines if the defects are located on the surface or within the interior of the object by comparing the scattered images of first and second wavelengths, and also acquires the depth information of the defects within the interior of the object from the scattered image of the second wavelength. However, since the depth at which the defects are located within the interior of the object is detected by use of only the second wavelength light (the optical information by the first wavelength light is removed and the information of only the internal defects is extracted), the resolution of the depth detection is determined by the optical image forming performance. Thus, since the wavelength of the radiated light is about 1 .mu.m, the defects in the near-surface region of the silicon substrate cannot be selectively detected. In addition, the above JP-A publication does not describe any means for solving the problem that the second wavelength light is attenuated in the interior of the object being tested.
Moreover, in Japanese Patent Laid-open JP-A-2-61540 disclosed is another defect inspecting apparatus capable of easily determining if foreign particles are located on the upper or lower side of the surface of a transparent, flat object (such as thin film or relatively thin transparent plate) and detects the sizes of the foreign particles. This apparatus radiates first and second light beams of different transmission factors on the object, and determines if the foreign particles are located on the incidence surface or the opposite side of the object by comparing the intensities of the scattered light signals. However, the above JP-A publication does not describe anything about the detection of the depth at which the defects are located within the interior of the object being inspected.
Before detecting the foreign particles on the surface of the sample and the foreign particles or crystal defects in the near-surface region, we need standard samples for correcting the detection sensitivity of measuring equipment and so on. A standard sample for detection of foreign particles on the surface, or standard particles, of which the refractive index and size are known, deposited on a substrate surface, is disclosed in Japanese patent Laid-open JP-A-5-340884. In addition, a standard sample of particle images reduced and transferred onto a transparent substrate is disclosed in Japanese Patent Laid-open JP-A-5-332913. However, there is no document which describes a standard sample for detecting the defects located in the near-surface region of the sample, or located within the sample at a depth that is shorter than the wavelength of the irradiation light.